A variable valve timing control system for an engine is configured such that a crankshaft of the engine is coupled to a drive cam by a driving power transmission mechanism such as a chain, the drive cam rotates in association with rotation of the crankshaft and a pivot cam mechanism causes an intake valve and an exhaust valve to reciprocate in association with rotation of the drive cam. The variable valve timing control system is configured to change a pivot range and a pivot phase of the pivot cam mechanism to change lift characteristics of the intake valve and the exhaust valve (see patent document 1). The variable valve timing control system is disposed inside a cylinder head cover above a cylinder.
A pivot cam mechanism for a variable valve timing control system disclosed in Japanese Laid-Open Patent Application Publication No. Hei. 6-74010 includes a pivot cam configured to contact a tappet of a valve, a pivot arm configured to contact a drive cam, and a rotatable pivot camshaft. The pivot cam mechanism further includes a rotary member configured to rotate the pivot cam relative to the pivot camshaft. In the pivot cam mechanism, the rotary member applied with a driving power from a drive means causes the pivot cam to rotate. The rotation changes a relative angle around the pivot camshaft between the pivot arm and the pivot cam, changing the lift characteristics.
In the variable valve timing system, the rotary member configured to rotate the pivot cam is positioned near the valve. To be specific, the rotary member is positioned between the pivot cam and the pivot arm. A cylinder member is provided to directly drive the rotary member. The cylinder member is also positioned near the rotary member. To be specific, the cylinder member is disposed inside a casing of the engine and positioned in the vicinity of the valve, such as in the rotary member. In a structure in which the cylinder member and the rotary member are positioned in the vicinity of the valve in this way, design of the cylinder member and the rotary member is restricted to a considerable extent, because the shapes and arrangement of them affect each other. For this reason, flexibility of the design of the engine is lessened.